I had to do research on it for school, which was a few years ago so I can't remember it exactly but it's something like forcing radioactive materials together releases a huge amount of energy.
It usually uses a massive explosion in the initial stage to create the energy required to fuse the particles together. In effect they are a two stage device, a fission device that creates the energy for the much larger fusion device.
Tokomaks, basically the reactors for Fusion system (toroidal chamber with magnetic coils) work by using high energy plasma contained inside a toroidal magnetic field to force a combination of either Tritium or Heluim3 and Deuterium together to create the Fusion bursts that produce the energy. The big sticking point being that
a). To start the process you have to heat the plasma to 100million degrees C, this alone take s quite a considerably level of energy
b). The theory is that once started the process should be self sustaining provided you keep providing the reactor with Tritium or Heluim3 and Deuterium. As you add these components the plasma cools so part of the energy created from the fusion will have to be used to heat the plasma, as things stand right now they are incapable of creating enough energy from the process to keep the plasma heated.
You also have to consider some other factors
Deuterium or heavy hydrogen is massively abundant within the Earth's seas
3H Helium 3 on the other hand is very rare on Earth although it is believed their may be a large amount of it on the moon
Tritium is also occurs very rarely in nature, the comically ironic thing is that Tritium is sometimes a by product of Nuclear Fission
In effect the current Fusion programs use these components because they produce a lot of energy. They are the closest we have, at the moment, to producing enough energy just to keep the reaction system operating, forget powering anything else. To make Fusion viable we need to make the entire process more efficient and it's not so that we can actually draw power from the device whilst having enough left over to keep the reaction going but rather so we can start to use more commonly available particles to produce the reaction such as Deuterium and Deuterium which when fused produces less energy than using D - H3 and D - T
As for ITER, as a Fusion reactor for now it is nothing more than another big toy for experimenting with, will it produce power, yes it will, will the power be viable, no it won't, chances are the experiment won't even last long enough to ever become a viable power source since the neutron bombardment from the reaction will actually end up destroying the materials from which the reactor is made.
Will we ever have Fusion... no, I don't think so not unless we find another abundant particle that produces a huge amount of energy when fused. Either that or a particle that can be fused at much much lower energy levels needs to be found. Right here and right now the only options for the foreseeable future require fusing a combination of rare particles with common particles and then trying to extract the energy at an efficiency as close to 100% as possible and the results just don't stand up.
We need to plough more money in to fission as right here and right now it DOES produce power and a vast amount of it. We need to improve the efficiency of fission and the waste processing simply because it does work and it could work even better.
